1. Field of the Invention
The present invention relates to a control device for a robot, such as a legged mobile robot.
2. Description of the Related Art
As a technique for controlling a legged mobile robot, such as a bipedal walking robot, there has been generally known so-called position control proposed in, for example, Japanese Patent Application Laid-Open No. 5-305579 by the applicant of the present application. According to the disclosed technique, the desired displacement amount of each joint of a robot is sequentially determined on the basis of a desired motion of the robot (e.g., the trajectory of a desired position of the distal end of a leg of the robot), and then the actuator of each joint is drivably controlled so as to cause an actual displacement amount to coincide with the desired displacement amount.
According to the position control described above, the actual displacement amount of a joint is maintained to be constant as long as the desired displacement amount of the joint remains constant, thus inconveniently presenting poor flexibility in response to a change in an external force acting on the robot.
As a solution to the inconvenient inflexibility, there has been known a technique disclosed in, for example, Japanese Patent Application Laid-Open No. 2007-160446. According to the technique, a spring member is interposed between a joint and an actuator to impart flexibility to the joint, and a desired driving force for each joint to accomplish a desired motion of a robot is determined by inverse dynamics calculation so as to control the actual driving force of each joint to the desired driving force thereof by so-called force control.
According to the technique based on the force control, as disclosed in the aforesaid Japanese Patent Application Laid-Open No. 2007-160446, the desired rotational angle, angular velocity and angular acceleration of each joint for accomplishing a desired motion of the robot are determined, and then the inverse dynamics calculation is carried out on the basis of these desired values, thereby determining the feed-forward value of a desired driving torque to be imparted to each joint. Further, according to the technique disclosed in Japanese Patent Application Laid-Open No. 2007-160446, the desired driving torque for each joint is determined by adding a feed-back manipulation torque, which is for converging the rotational angle of each joint of the robot to a desired value, to the feed-forward value.
Thus, according to the technique disclosed in Japanese Patent Application Laid-Open No. 2007-160446, the actual motion of each joint of the entire robot will be significantly susceptible to restrictions based on the desired values of the rotational angle, the angular velocity and the angular acceleration of each joint. Hence, as long as the desired values remain constant, the desired driving torque for each joint will be inconveniently adjusted so as to maintain the motion of each joint to be constant even if an external force acting on the robot changes.
Therefore, according to the technique disclosed in Japanese Patent Application Laid-Open No. 2007-160446, the actual motion of each joint of the robot will be undesirably least flexible, possibly making it difficult to maintain a stable posture of the robot when, for example, an external force acting on the robot suddenly changes.